In power semiconductor devices, energy loss during power conversion is reduced, and energy saving can be expected. So far, the performance of power semiconductor devices has been improved using silicon (Si) semiconductors, but a situation has been reached in which further performance improvement cannot be anticipated for silicon semiconductor devices because of the Si physical property limit.
On the other hand, silicon carbide (SiC) has excellent physical properties, a dielectric breakdown electric field strength about 10 times that of Si and a forbidden band width and a thermal conductivity about 3 times those of Si, and further performance improvement of power semiconductor devices can be expected, and rapid spread of SiC semiconductors using this is required.
SiC has many crystal types, and for a SiC substrate for a SiC semiconductor device expected as a next-generation semiconductor device for power conversion, one having a 4H type crystal structure is generally used. In addition, from the viewpoint of using a step control epitaxy technique, a 4H—SiC substrate having an off-angle is the mainstream. When an epitaxially grown layer can be effectively formed on a low off-angle SiC substrate having an off-angle of less than 5°, particularly less than 2°, the manufacturing cost of a SiC semiconductor device can be reduced, and with this, wide effective utilization in society is expected.
Usually, when a SiC substrate is cut from a SiC ingot, a predetermined off-angle from the (0001) Si face is formed on the SiC substrate. The cut SiC substrate is surface-processed by polishing or the like and then used in a state of an epitaxial wafer in which an epitaxially grown layer is formed on the substrate surface.
Here, when a rough portion is present on the surface of the epitaxial wafer, the performance of a semiconductor structure fabricated thereon decreases, and the reliability decreases. Particularly, the decrease in the reliability of the oxide film is significant, and the improvement of the surface flatness of the epitaxial wafer is essential for the reliability improvement of a MOSFET or the like.
A SiC substrate cut from a SiC ingot is subjected to hydrogen etching before the formation of an epitaxially grown layer for the purpose of removing surface damage such as polishing flaws to flatten the surface, and the like, and a method for the flatness improvement is also proposed (for example, see PTL 1).